1. Field of the Invention
The present invention relates to a gas detector and, more particularly, to a gas detector that utilizes an electric field to assist in the collection and removal of gas molecules.
2. Description of the Related Art
A semiconductor-based gas detector is a device that is sensitive to the presence of a gas species. When a gas detector is exposed to a gas species, the gas detector collects gas molecules and then measures the number of collected gas molecules to determine the concentration of the gas species. Carbon diode and other gas species can be detected by a gas detector.
FIGS. 1A-1C show views that illustrate a first example of a conventional gas detector 100. FIG. 1A shows a plan view. FIG. 1B shows a cross-sectional view taken along line 1B-1B of FIG. 1A, while FIG. 1C shows a cross-sectional view taken along line 1C-1C of FIG. 1A. As shown in FIGS. 1A-1C, gas detector 100 includes a p− substrate 110, a shallow trench isolation region STI that is formed in substrate 110, and an NMOS transistor 114 that is formed in and on substrate 110.
NMOS transistor 114, in turn, includes spaced-apart source and drain regions 116 and 118 that are formed in substrate 110, and a channel region 120 of substrate 110 that lies between the source and drain regions 116 and 118. The source and drain regions 116 and 118, in turn, each include an n+ region and an NLDD region.
In addition, NMOS transistor 114 includes a gate dielectric layer 122 that touches the top surface of substrate 110 over channel region 120, a gate 124 that touches the top surface of gate dielectric layer 122 over channel region 120, and a side wall spacer 126 that touches the side wall of gate 124.
Gate 124 is implemented with a material that has a high permeability to the gas species to be detected. For example, lanthanum oxide, tin oxide, indium oxide, and zink oxide are materials which have a high permeability to carbon dioxide. Other materials are well known to have high permeabilities to other gas species.
As further shown in FIG. 1, gas detector 100 also includes a first dielectric layer 130 that touches the top surface of substrate 110, and a number of contacts 132 that extend through first dielectric layer 130. The contacts 132 make individual electrical connections to source region 116, drain region 118, gate 124, and a p+ region of substrate 110.
In addition, gas detector 100 includes a number of metal traces 134 that touch the top surface of first dielectric layer 130, and a second dielectric layer 136 that touches the top surface of first dielectric layer 130 and the metal traces 134. The metal traces 134 make electrical connections to the contacts 132. Gas detector 100 further includes a window opening 140 that extends through the first and second dielectric layers 130 and 136 to expose the top surface of gate 124.
In operation, gas detector 100 begins with a calibration step, which is performed in an environment that is known to be free of, or have an insignificant concentration of, the to-be-measured gas species. The calibration step is utilized to determine a bias voltage for gate 124, which is used in a subsequent measurement step.
Gas detector 100 can be calibrated by first applying ground to substrate 110 and source region 116, a VCC voltage to drain region 118, and an initial calibration voltage to gate 124. Once the voltages have been applied, the magnitude of the source current is measured. The initial calibration voltage is selected to ensure that a sub-threshold current flows out of source region 116.
Following this, the calibration voltage is incrementally increased, and the magnitude of the source current is re-measured. The process of incrementally increasing the calibration voltage and re-measuring the magnitude of the source current is repeated a number of times until the magnitude of the source current increases substantially, indicating the turn on of NMOS transistor 114.
After the source current has increased substantially, the process of incrementally increasing the calibration voltage and re-measuring the magnitude of the source current ends. Next, a calibration voltage is selected to be the bias voltage from the calibration voltages which were used to generate the source currents. For example, the calibration voltage selected to be the bias voltage can be the calibration voltage which lies just below the turn on voltage of NMOS transistor 114.
Once the bias voltage for gate 124 has been selected, the calibration step ends and a collection step begins. The collection step begins, for example, by grounding p− substrate 110, source region 116, and drain region 118, and electrically floating the gate 124 for a predetermined period of time.
When gas detector 100 is exposed to the gas species, random gas molecules of the gas species enter window 140 and hit the exposed top surface of gate 124. When a gas molecule hits the exposed top surface of gate 124, the gas molecule can bounce away from, or stick to, the exposed top surface of gate 124.
Due to the high permeability of the material used to form gate 124, a number of gas molecules that stick to the exposed top surface of gate 124 are absorbed by gate 124. The gas molecules that stick to gate 124 and are absorbed into gate 124 change the work function of the material used to form gate 124 which, in turn, has the effect of placing a positive charge on gate 124.
After the predetermined period of time, the collection step ends and a measurement step begins to determine the number of gas molecules which have been collected. The measurement step begins, for example, by applying the VCC voltage to drain region 118, grounding substrate 110 and source region 116, and applying the bias voltage to gate 124.
The total charge on gate 124 is the combination of the bias voltage and the effective charge placed on gate 124 by the gas molecules. As a result, when gas molecules have been collected, the total charge on gate 124 is greater than the bias voltage which, in turn, causes the source current to be larger than when no gas molecules have been collected. Thus, by evaluating the increase in source current when compared to the source current associated with the bias voltage, the effective charge placed on gate 124 by the gas species can be determined or accurately estimated.
The concentration of the gas species that corresponds with the increase in source current or the effective charge placed on gate 124 can then be determined by referencing a look-up table, where the entries in the look-up table are experimentally determined from a series of increased source currents and known gas concentrations.
FIGS. 2A-2D show views that illustrate a second example of a conventional gas detector 200. FIG. 2A shows a plan view. FIG. 2B shows a cross-sectional view taken along line 2B-2B of FIG. 2A, while FIGS. 2C and 2D both show a cross-sectional view taken along line 2C-2C of FIG. 2A. Gas detector 200 is similar to gas detector 100 and, as a result, utilizes the same reference numerals to designate the structures which are common to both detectors.
As shown in FIGS. 2A-2D, gas detector 200 differs from gas detector 100 in that gas detector 200 eliminates window 140 and utilizes a floating gate structure 224 in place of gate 124. Floating gate structure 224, which is conductive and electrically isolated from all other conductive structures, includes a lower floating gate 230 that touches gate dielectric layer 122 and first dielectric layer 130, an upper floating gate 232 that touches second dielectric layer 136, and a vertical connection structure 234 that electrically connects upper floating gate 232 to lower floating gate 230, and extends through the first and second dielectric layers 130 and 136.
Lower floating gate 230 can be implemented with polysilicon, and upper floating gate 232 can be implemented with a conventional metal trace material that has no or a very low permeability to the gas species to be detected. The upper and lower portions of vertical conductive structure 234 can be implemented with a conventional via/contact material that has no or a very low permeability to the gas species to be detected, while the wider middle section of vertical conductive structure 234 can be implemented with a conventional metal trace material that has no or a very low permeability to the gas species to be detected.
Gas detector 200 also differs from gas detector 100 in that gas detector 200 has an inter-gate dielectric 240, such as oxide-nitride-oxide (ONO), that touches the top surface of lower floating gate 230, and a control gate 242 that touches inter-gate dielectric 240 and lies over a portion of the top surface of lower floating gate 230.
In addition, rather than a contact 132 making an electrical connection with gate 124, the contact 132 instead makes an electrical connection with control gate 242. (Rather than utilizing dielectric 240 and control gate 242 as shown in FIG. 2C, a heavily doped region 244 that touches gate dielectric 122 and lies below a portion of lower floating gate 230 can alternately be formed as the control gate as illustrated in FIG. 2D. Although not shown, a contact 132 makes an electrical connection to doped region 244.)
Gas detector 200 further differs from gas detector 100 in that gas detector 200 includes a third dielectric layer 250 that touches the top surface of second dielectric layer 136 and upper floating gate 232. Gas detector 200 additionally differs from gas detector 100 in that gas detector 200 includes a detection structure 252 that touches the top surface of third dielectric layer 250.
Detection structure 252, which is electrically isolated from all other conductive structures, is implemented with a material that has a high permeability to the gas species to be detected. For example, lanthanum oxide, tin oxide, indium oxide, and zink oxide are materials which have a high permeability to carbon dioxide. Other materials are well known to have high permeabilities to other gas species.
The operation of gas detector 200 begins by calibrating gas detector 200 to determine a bias voltage for control gate 242 (or doped region 244). Gas detector 200 can be calibrated in the same manner as gas detector 100, except that the source currents are measured in response to placing voltages on control gate 242 (or doped region 244). The voltages placed on control gate 242 (or doped region 244), in turn, are capacitively coupled to floating gate structure 224. Due to the capacitive coupling, the bias voltage selected for control gate 242 (or doped region 244) is slightly larger than the bias voltage selected for gate 124.
Once the bias voltage for control gate 242 (or doped region 244) has been selected, the calibration step ends and a collection step begins. The collection step begins, for example, by grounding p− substrate 110, source region 116, drain region 118, and control gate 224 (or doped region 244) for a predetermined period of time.
When gas detector 200 is exposed to the gas species, random gas molecules of the gas species hit the exposed surface of detection structure 252. When a gas molecule hits the exposed surface of detection structure 252, the gas molecule can bounce away from, or stick to, the exposed surface of detection structure 252.
Due to the high permeability of the material used to form detection structure 252, a number of gas molecules that stick to the exposed surface of detection structure 252 are absorbed by detection structure 252. The gas molecules that stick to detection structure 252 and are absorbed into detection structure 252 change the work function of the material used to form detection structure 252 which, in turn, has the effect of placing a positive charge on detection structure 252.
After the predetermined period of time, the collection step ends and a measurement step begins to determine the number of gas molecules which have been collected. The measurement step begins, for example, by applying the VCC voltage to drain region 118, grounding p− substrate 110 and source region 116, and applying the bias voltage to control gate 224 (or doped region 244).
The total potential on floating gate structure 224 is defined by the voltage on control gate 242 (or doped region 244) and the effective charge placed on detection structure 252 by the gas molecules, both of which are capacitively coupled to floating gate structure 224. As a result, when gas molecules have been collected, the total potential on floating gate structure 224 is greater than the capacitively coupled potential of the bias voltage which, in turn, causes the source current to be larger than when no gas molecules have been collected. Thus, by evaluating the increase in source current when compared to the source current associated with the gate bias voltage, the effective charge placed on detection structure 252 by the gas species can be determined or accurately estimated.
The concentration of the gas species that corresponds with the increase in source current or the effective charge placed on detection structure 252 can then be determined by referencing a look-up table, where the entries in the look-up table are experimentally determined from a series of increased source currents and known gas concentrations.
One of the advantages of gas detector 200 over gas detector 100 is that third dielectric layer 250 provides an environmental barrier to the components that lie below third dielectric layer 250. By forming third dielectric layer 250 to be relatively thin, most of the effective charge placed on detection structure 252 can be capacitively coupled to floating gate structure 224.
In addition to using a transistor-based gas detector, resistor-based gas detectors can alternately be used. This is because in addition to effectively adding a positive charge to a material, gas molecules that stick to and are absorbed by a high permeability material also change the conductivity of the material.
FIGS. 3A-3C show views that illustrate a third example of a conventional gas detector 300. FIG. 3A shows a plan view. FIG. 3B shows a cross-sectional view taken along line 3B-3B of FIG. 3A, while FIG. 3C shows a cross-sectional view taken along line 3C-3C of FIG. 3A. Gas detector 300 is similar to gas detector 100 and, as a result, utilizes the same reference numerals to designate the structures which are common to both detectors.
As shown in FIGS. 3A-3C, gas detector 300 differs from gas detector 100 in that gas detector 300 utilizes a resistive structure 310 in lieu of gate 124, and a dielectric layer 312 in lieu of dielectric layer 122. Gas detector 300 also differs from gas detector 100 in that gas detector 300 eliminates the source and drain regions 116 and 118, and utilizes the contacts 132 to make connections to opposite sides of resistive structure 310. Resistive structure 310 can be identical to gate 124, while dielectric layer 312 can be thicker than gate dielectric layer 122.
Gas detector 300 can be calibrated by grounding substrate 110, applying a set of voltages to the opposite sides of resistive structure 310, and then measuring a baseline current through resistive structure 310. Gas detector 300 can collect gas molecules by electrically floating resistive structure 310, and grounding p− substrate 110. During collection, gas molecules that stick to, and are absorbed by, resistive structure 310 change the conductivity of resistive structure 310.
Gas detector 300 can measure the number of collected gas molecules by grounding substrate 110, applying the set of voltages to the opposite sides of resistive structure 310, and then measuring a current through resistive structure 310. Thus, by evaluating the change in current through resistive structure 310, the number of gas molecules collected by resistive structure 310 can be determined or accurately estimated. The concentration of the gas species that corresponds with the change in current can then be determined by referencing a look-up table, where the entries in the look-up table are experimentally determined from a series of currents and known gas concentrations.
Although gas detectors 100, 200, and 300 can be utilized to detect a number of gas species, there is a need for additional structures for detecting the presence of a gas species.